This invention relates to pneumatic tires, and more particularly relates to pneumatic tires having a reinforcing belt in the area of the carcass which underlies the tread of the tire.
As used in this disclosure with reference to a tire, the terms "circumferential" should be understood as referring to the directions around the circumference of the tire. The terms "axial" and "axially" should be understood as referring to the directions parallel to the axis of revolution of the tire and perpendicular to the median equatorial plane of the tire. The terms "radial" and "radially" should be understood as referring to the directions toward or away from the axis of revolution of the tire.
The terms described above may also be used with reference to an endless, hooplike belt structure adapted to underlie the tread of a tire, and when they are so used they should be understood with reference to the circumference, the median equatorial plane and the axis of revolution of the belt itself. In a tire which incorporates a belt, the circumferential, axial and radial directions of the belt will normally be identical with the corresponding directions of the tire.
The tread region of a tire carcass should have substantial rigidity or resistance to deformation in the circumferential and axial directions. As the rigidity of the tread region of the carcass in the circumferential and axial directions is increased, the tread wear and handling characteristics of the tire improve. To provide such increased rigidity, it has long been known in the art to provide an endless, hooplike reinforcing belt in the crown region of the carcass which underlies the tread. Such reinforcing belts are expecially useful in the so-called "radial ply" type of tire. In the radial ply type of tire, the cords of the carcass or body plies of the tire extend substantially radially in the sidewalls of the tire and extend substantially axially in the crown region of the tire. Because the body ply cords of "radial ply" tires do not reinforce the crown or tread regions of such tires in the circumferential direction to any substantial degree, "radial ply" tires almost universally incorporate reinforcing belts.
The reinforcing belts of the prior art consist of one or more plies of rubber coated cords. These plies are constructed as endless hoops which extend circumferentially in the crown region of the carcass radially outwardly of the carcass plies but radially inwardly of the tread.
If a single belt ply is used, the cords of that ply are oriented so that they extend substantially circumferentially or substantially parallel with the median equatorial plane of the tire.
If a plurality of plies are used, the prior art teaches that they are to be provided in pairs of plies, the cords of each ply of a pair being formed from the same material as the cords of the other ply of the same pair. The cords of each ply of such a pair intersect the median equatorial plane of the tire at an angle, but the pairs of plies are arranged so that the cords of one ply of the pair intersect the median equatorial plane at an angle which is equal in magnitude but opposite in direction from the angle at which the cords of the other ply of the same pair intersect the median equatorial plane. Thus, the cords of each ply of such a pair may be said to be "crossed" with respect to the cords of the other ply of the same pair. The two plies constituting each such pair are positioned immediately adjacent to one another.
For example, U.S. Pat. No. 3,850,219 teaches the use of one pair of steel cord plies. The cords of one of these plies intersect the median equatorial plane of the tire at an angle of from 16.degree. to 30.degree. in one direction. The cords of the other one of these plies intersect the median equatorial plane of the tire at an angle of the same magnitude but in the opposite direction. U.S. Pat. No. 3,126,042 teaches the use of two pairs of plies: a pair of steel cord plies and a pair of textile cord plies. The pair of textile cord plies is positioned radially outwardly of the pair of steel cord plies. U.S. Pat. No. 2,982,328 also teaches the use of pairs of plies.
Other reinforcing belt structures are taught by U.S. Pat. Nos. 3,793,612 and 3,786,851, and by French Pat. No. 1,586,370.
All of the aforementioned reinforcing belt structures reinforce the crown or tread region of the tire in both the lateral and circumferential directions to some extent. However, none of these structures combines, to the optimum extent, the desirable rigidty of the crown region with the soft riding characteristics, light weight and low cost of manufacture which are also desirable.
Further, reinforcing belt structures are subject to the phenomenon known as "belt edge separation". Belt edge separation occurs when the edges of the belt plies become displaced radially outwardly from their normal position during high speed operation of the tire and thus become separated from the underlying carcass plies. Various expedients have been suggested to eliminate belt edge separation. For example, the aforesaid U.S. Pat. No. 3,850,219 and the aforesaid French Pat. No. 1,586,370 teach the use of so-called "cap plies" as a part of the belt structure. The "cap plies" are positioned radially outwardly of the other belt plies, and consist or rubber coated nylon cords which extend substantially parallel to the median equatorial plane of the tire. These "cap plies" help to prevent belt edge separation by physically restraining the other plies of the reinforcing belt against radially outward movement. However, a further increase in resistance to belt edge separation beyond the degree of resistance provided by the cap plies alone is still desirable.